Perimeter of Triangle

The Perimeter of any two dimensional figure is defined as the distance around the figure. We can just calculate perimeter of any closed shape just adding up the length of each of the sides. In this article will first learn about what is perimeter, how to find the perimeter of different types of triangles when all side lengths are known. At last solved examples help you get a more clarification on the topic.

What is the Perimeter of a Triangle

The sum of the lengths of the sides is the perimeter of any polygon. In case of triangle,

Perimeter = Sum of the three sides.

Always include units in the final answer. If the sides of the triangle are measured in centimeters then finals answer should also be in centimeters.

How to Find the Perimeter of a Triangle

Below table helps us to understand how to find the perimeter of different triangles- Equilateral triangle, Isosceles triangle and Scalene triangle.

Equilateral Triangle

Isosceles Triangle

Scalene Triangle

P = 3.I

P = 2.I + b

P = a + b + c

Perimeter of a Triangle
Perimeter of a Triangle
Perimeter of a Triangle

Where a, b, c and l are the side lengths, and P = Perimeter.

This formula implies to find the perimeter of a triangle, add the lengths of all of its 3 sides together. If A, B and C are the side measures, and X is perimeter then

Perimeter Of Triangle


Let us consider some of the examples on perimeter of a triangle:

Example 1: Find the perimeter of a polygon whose sides are 5 cm, 4 cm and 2 cm?

Solution: Let,

a = 5 cm

b = 4 cm

c = 2 cm

Perimeter = Sum of all sides = a + b + c = 5 + 4 + 2 = 11

Therefore, the answer is 11 cm.

Example 2: Find the perimeter of a triangle whose each side is 10 cm.

Solution: Since all three sides are equal in length, is an equilateral triangle.

Perimeter = 30 cm. Answer!

Example 3: What is missing side length of a triangle whose perimeter is 40 cm and two sides are 10 cm each.

Solution: Given,

Perimeter = 40 cm

Length of two sides are same i.e. 10 cm, is an isosceles triangle.

Using formula: P = 2l + b

40 = 2 * 10 + b

40 = 20 + b

or b = 20

Missing side length is 20 cm.

To learn more about triangles and related topics in Geometry, register with BYJU’S today and sharpen your skills.

Read More:

Practise This Question

The region shaded purple (OAB) is known as :